1. Field of the Invention
The present invention generally relates to methods and apparatuses for manufacturing a plate-like body, and more particularly to a method and apparatus for manufacturing a plate-like body formed of two members stuck together.
2. Description of the Related Art
Recently, with the digitization of information apparatuses and the accompanying rapid development of multimedia apparatuses, there has been a sharp increase in the amount of processed information (data), thus requesting further increases in the capacities of information recording media.
In the case of an optical information recording medium, for instance, the DVD (digital versatile disk) has come to the front as a next-generation optical information recording medium replacing the CD (compact disk), which has been widely used. The DVD, which has the same diameter as the CD, can record approximately seven times as much data as the CD due to technological improvements in shortening the wavelength of a laser serving as the light source of an optical pickup.
While the CD is formed of a single substrate 1.2 mm in thickness, the DVD is formed of two 0.6 mm-thick substrates stuck together, so that information can be recorded on and reproduced from the CD and the DVD in the same apparatus.
Accordingly, in the process of manufacturing an optical information recording medium formed of two substrates stuck together, such as the DVD, a process of sticking the two optical disk substrates together is essential At the time of sticking the two substrates together, it is important that no air bubbles be allowed to enter or remain in an adhesive agent or between the two substrates. This is because it is probable that air bubbles would become like lenses to refract a laser beam emitted from an optical pickup at the time of reading information, thereby preventing the laser beam from being emitted onto a desired position and causing information reading and reproduction failures. Further, in the case of an optical recording medium of a phase-change recording type, air bubbles may expand through laser emission in the laser annealing process (initialization process) performed after the sticking process so as to damage the recording layer of the recording medium, thus generating a serious defect in the quality of the recording medium. That is, the existence of air bubbles is one of the defect factors in the product life and the physical and electrical characteristics of the optical information recording medium.
A variety of technologies or inventions have been so far proposed on methods or apparatuses for controlling the above-described presence of air bubbles.
Japanese Laid-Open Patent Application No. 9-198720, for instance, discloses a method of sticking disks together so that air bubbles entering between the superimposed disks are removed more satisfactorily (hereinafter this method may be referred to as the first prior art). According to the first prior art, superimposed disks are sandwiched between upper and lower press plates fixed to a shaft part in a hermetically sealed space formed in the main body of a pressurizing device by being covered by a-chamber of a rigid body. When the hermetically sealed space is evacuated to vacuum through an evacuation tube, external pressure added to the chamber deforms an O-ring sealing the chamber, so that the superimposed disks are pressurized by the deformation of the O-ring. That is, the air bubbles in an adhesive agent between the superimposed disks are expelled to the outside by the above-described evacuation.
Japanese Laid-Open Patent Application No. 11-66645, for instance, discloses a technology employing evacuation of a local part of two substrates stuck together through an adhesive agent (hereinafter this technology may be referred to as the second prior art). According to the second prior art, the adhesive agent is provided in a circular ring form on the surface of the lower one of the two substrates, and with the upper substrate being held by a section in the vicinity of its periphery, the two substrates are brought closer to each other. Then, before the upper substrate comes into contact with the adhesive agent provided in the circular ring form on the surface of the lower substrate, gas is evacuated from the internal space surrounded by the surface of the upper substrate, the surface of the lower substrate, and the adhesive agent so that pressure inside the internal space is lower than atmospheric pressure in the external space. In this state, the upper substrate is brought into contact with the adhesive agent provided in the circular ring form on the surface of the lower substrate, thereby reducing generation of air bubbles.
Further, Japanese Laid-Open Patent Application No. 2000-290602 discloses a technology of reducing generation of air bubbles by forming an electric field between two substrates and causing an adhesive agent to have a reduced contact area by tapering off the top parts of the liquid film spots of the adhesive agent by using the attraction of the electric field (hereinafter this technology may be referred to as the third prior art).
However, in the first prior art, the vacuum chamber has a large volume so as to cover the substrates. Therefore, the pressurizing apparatus is required to have a large-scale configuration so as to increase its production cost accordingly. Further, since this type of vacuum chamber has a large internal volume, it is necessary to increase suction force for evacuation and it takes a long time before desired pressure conditions are achieved by evacuation, thus resulting in poor operation efficiency.
According to the second prior art, it is possible to produce a certain effect by making the pressure inside the internal space surrounded by the surfaces of the upper and lower substrates and the adhesive agent lower than the atmospheric pressure in the external space, that is, by locally creating a vacuum atmosphere. However, at the time of the above-described evacuation, negative pressure is suddenly generated in a part around the center hole of the upper substrate, which part corresponds to the center part of the internal space surrounded by the surfaces of the upper and lower substrates and the adhesive agent, so that the adhesive agent may be sucked in. Such a case may result in insufficient joining in sticking together the two substrates, which is the primary object prior to ensuring removal of air bubbles mixed into the adhesive agent.
Further, according to the third prior art, the adhesive agent is allowed to have a reduced contact area by using the attraction of the electric field, thus producing a significant effect in reducing generation of air bubbles. According to the third prior art, however, it is necessary to take a certain measure against the electric field, thus resulting in an increase in the production cost of the apparatus. Further, normally, static electricity elimination and dustproof air blowing are performed as a measure against adhesion of dust to substrates by static electricity (anti-contamination measure) before the adhesive agent is provided. In the third prior art, however, dust in the sticking apparatus is attracted to the substrates undesirably as a result of charging the substrates. Further, the charged substrates are required to be discharged. This increases the cost of the apparatus and the number of operations in the sticking process, thus decreasing productivity.